Circuit board having a plated through hole through a conductive pad

ABSTRACT

In some embodiments, an interconnectable circuit board may include one or more of the following features: (a) a first electrically conductive pad located on a top of the circuit board, (b) a plated through hole on the conductive pad which passes through the circuit board, (c) a second electrically conductive pad coupled to the plated through hole; the second conductive pad capable of being electrically connected to a third electrically conductive pad attached to a top of a second interconnectable circuit board, (d) cut marks indicating safe locations for separating the circuit board, and (e) a second cut mark adjacent to the first cut mark where the area between the first and second cut mark can be utilized to make a safe cut through the circuit board.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/944,610, filed on Jul. 17, 2013, which is a divisional application ofU.S. application Ser. No. 13/190,639, filed on Jul. 26, 2011, now U.S.Pat. No. 8,500,456 and which is a continuation application of U.S.application Ser. No. 12/406,761, filed on Mar. 18, 2009, now U.S. Pat.No. 8,007,286, and which claims the benefit of U.S. Application Ser. No.61/037,595, filed on Mar. 18, 2008, and U.S. Application Ser. No.61/043,006, filed on Apr. 7, 2008, the content of all of which is hereinincorporated by reference in its entirety.

FIELD OF THE INVENTION

Embodiments of the present invention relate to the design and method ofinterconnecting printed circuit boards. Particularly, embodiments of thepresent invention disclose a method for making long or continuouscircuit strips, grids, matrices or links structures using the same. Moreparticularly, embodiments of the present invention disclose theconstruction of layered, semi-flexible interconnected circuits for usewith high powered LEDs in Solid-State Lighting (SSL) applications.

BACKGROUND OF THE INVENTION

In many electronic systems and products, multiple printed circuit boardsare used with connectors, harnesses and cables making circuitconnections between them. Interconnection of circuit boards may beaccomplished by the use of surface mount connectors, wires or wirecables, flex circuit strips, edge connectors, wire pins or shunts.Typically, these connections carry power from one circuit to another, aswell as conductors for electronic communication, sensing and control.While there are many types of connections, there are limitations anddisadvantages to most of them.

In some applications it is desirable to connect one board to anotherover a short distance, with minimal numbers of components and materialemployed in the connection. Further, the type and number ofinterconnections has a strong effect on reliability. Conventional cablesand harnesses employ wires, terminals and pins, which must be joinedtogether mechanically. Failure in any one of these joints or reductionin conductivity due to mechanical effects, corrosion or fracture willcause failure of the circuit. For this reason, solder joints are oftenused because of their reliability and permanence.

Solder connection between circuit boards, while being reliable, usuallyrequire the spanning of the distance between boards or conductors with aconductor such as a pin (e.g., a shunt being a larger form of pin) or awire. Pins are rigid and sometimes present unwanted stresses on theboard and connection locations (e.g., the pads and holes). Secondarymechanical structures are added to reduce and control stresses. The pinsthemselves must be soldered to the board either manually (e.g., one at atime) or using special equipment. Press-in pins (e.g., pins, which relyon mechanical interference with conductors or pads) are sometimes usedwhen geometries are fixed and well controlled.

Wires are flexible but are more difficult to reliably solder join andlack the structure for mechanical linkage when this is required.Typically, wires are directly soldered onto boards and are insertedthrough holes or soldered onto an enlarged copper pad. Direct solderingof wires is often done manually or with the use of equipment specializedfor this purpose. Additional mechanical structures, called strainrelief, are required to prevent mechanical fatigue and fraying of thewire adjacent to the solder joint if any type of motion or vibration isanticipated.

A third type of interconnect, called a flex circuit, is particularlyadvantageous where multiple circuits are joined carrying small amountsof current in limited space. Flex circuits are typically made byprinting a thin metal conductive layer with a conductive pattern on ahighly flexible plastic substrate. To prevent damage to the thinconductive layer, an additional layer of plastic is laminated over theconductor to form a sandwich. Access to the conductors is provided viaholes in one or both of the plastic layers. Still, in order to gainrobustness at the connecting ends, mechanical connectors or solderedpins must be added to the design. Flex circuits usually do not add tothe mechanical stability or strength of the board-to-board connection.

For almost all of these connection methods described above, protectionof the connection from shorting contact, mechanical damage or ESD(electro-static discharge) requires an additional mechanical cover orcoating to be added after the solder connection, adding more complexityand cost to the implementation. Also, most of these interconnectionmethods present difficulties because of their mechanical sizes,geometries and lack of precise and flat mating surfaces for use onstrictly surface mount boards.

In various applications, such as production of high power solid-state(LED) lighting strips it is advantageous to have interconnectionsbetween circuit boards which are highly reliable, carry significantlevels of current or voltage without loss, are protected from mechanicaldamage and shorting, allow various shapes and geometries of connectionand are easy and efficient to apply.

Long lengths and or continuous runs of SSL circuit strips are desirablefor the reasons stated above. In addition, in order to make best use ofcircuit materials while distributing SSL components for area coverageand light direction, or to allow efficient shaping of the circuit toconform with the topology, curves and recesses of the structure it is tobe attached to, it is highly desirable have a reliable interconnectionbetween individual circuits.

In addition, the format of these semi-flexible continuous circuits isbeneficial to the manufacture of the continuous circuit or installationinto the final SSL fixture. Embodiments of the present inventiondescribed below conceive numerous methods to reduce manufacturing,installation and assembly costs. These system cost reductions furtherenable the adoption of SSL in a variety of applications, thus, reducingglobal energy consumption.

Solid-state lighting (SSL) refers to a type of lighting utilizinglight-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), orpolymer light-emitting diodes (PLEDs) as sources of illumination ratherthan electrical filaments, plasma (e.g., used in arc lamps such asfluorescent lamps) or gas. The term “solid-state” refers to the factlight in an LED is emitted from a solid object; a block of semiconductorrather than from a vacuum or gas tube, as is the case in traditionalincandescent light bulbs and fluorescent lamps. Compared to incandescentlighting, however, SSL creates visible light with reduced heatgeneration or parasitic energy dissipation, similar to fluorescentlighting. In addition, its solid-state nature provides for greaterresistance to shock, vibration and wear, thereby increasing its lifespansignificantly. Solid-state lighting is often used in area lighting,signage, traffic lights and is also used frequently in modern vehiclelights, train marker lights, etc.

BRIEF SUMMARY OF THE INVENTION

In some embodiments, an interconnectable circuit board may include oneor more of the following features: (a) a distal end having a firstelectrically conductive pad located on a top of the circuit board, (b) aplated through hole on the conductive pad which passes through aconductive layer of the circuit board and an insulative layer of thecircuit board, (c) a second electrically conductive pad coupled to theplated through hole, (d) a proximal end having a third electricallyconductive pad attached to the top of the circuit board, (e) alignmentmarks for use in aligning circuit boards when interconnecting them, (f)cut marks showing locations where the circuit board can be cut, (g)circuit paths electrically coupled the electrically conductive pads toprovide electrically interconnectivity between the circuit board and asecond circuit board, (h) a non-conductive solder repelling material ona surface of the circuit board, and (i) a fourth conductive pad forelectrically receiving electronic components.

In some embodiments, an apparatus for connecting circuit boards mayinclude one or more of the following features: (a) a first circuit boardhaving a first electrically conductive pad located on a top of the firstcircuit board, (b) a plated through hole on the first conductive padwhich passes through the circuit board, (c) a second electricallyconductive pad coupled to the plated through hole located on a bottom ofthe first circuit board, (d) a second circuit board having a thirdelectrically conductive pad on a top of the second circuit board,wherein the first circuit board can be placed upon the second circuitboard with the second conductive pad and third conductive pad aligned tocreate an electrical connection between the first circuit board and thesecond circuit board, (e) alignment marks located on the top of thefirst and second circuit boards for use in aligning the first and secondcircuit boards when interconnecting them, (f) cut marks showinglocations where the circuit board can be cut, and (g) a fourthconductive pad for electrically receiving electronic components.

In some embodiments, an interconnectable circuit board may include oneor more of the following features: (a) a first electrically conductivepad located on a top of the circuit board, (b) a plated through hole onthe conductive pad which passes through the circuit board, (c) a secondelectrically conductive pad coupled to the plated through hole; thesecond conductive pad capable of being electrically connected to a thirdelectrically conductive pad attached to a top of a secondinterconnectable circuit board, (d) cut marks indicating safe locationsfor separating the circuit board, and (e) a second cut mark adjacent tothe first cut mark where the area between the first and second cut markcan be utilized to make a safe cut through the circuit board.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a top and cut away view exposing layers of a circuit boardwith connection pads in an embodiment of the present invention;

FIG. 2A shows a top view of top board pads and holes in an embodiment ofthe present invention;

FIG. 2B shows a bottom view of top board pads and holes in an embodimentof the present invention;

FIG. 3 shows a top view of bottom board receiving pad geometry in anembodiment of the present invention;

FIG. 4A shows a top view of an assembled board prior to joining in anembodiment of the present invention;

FIG. 4B shows a top view of joined boards in an embodiment of thepresent invention;

FIG. 5 shows a top profile view of an overlapping joint between boardsin an embodiment of the present invention;

FIG. 6 shows a top profile view of potting material used to strengthenand protect connection joints in an embodiment of the present invention;

FIG. 7 shows a side view of a joint assembly of a flexible strip withcurvature in an embodiment of the present invention;

FIG. 8A shows a top view of the top board for a mid-length connection inan embodiment of the present invention;

FIG. 8B shows a bottom view of the top board for a mid-length connectionin an embodiment of the present invention;

FIG. 8C shows a top view of the bottom board for a mid-length connectionin an embodiment of the present invention;

FIG. 9 shows a top view of an overlapping connection used in strip arrayconstruction in an embodiment of the present invention;

FIG. 10 shows a top view of the construction of a two board by two boardgrid array in an embodiment of the present invention;

FIG. 11 shows construction of a larger grid using a two board by twoboard grid array in an embodiment of the present invention;

FIG. 12 shows another type of grid array wrapped around a cylindricalheat sink as an embodiment of the present invention;

FIG. 13A shows a top view of a board with a cut mark line for board orarray separation in an embodiment of the present invention;

FIG. 13B shows a top view of a board with a double line cut mark forboard or array separation in an embodiment of the present invention;

FIG. 13C shows the separation of two arrays of boards at one of the cutmarks in an embodiment of the present invention.

DETAILED DESCRIPTION

The following discussion is presented to enable a person skilled in theart to make and use the present teachings. Various modifications to theillustrated embodiments will be readily apparent to those skilled in theart, and the generic principles herein may be applied to otherembodiments and applications without departing from the presentteachings. Thus, the present teachings are not intended to be limited toembodiments shown, but are to be accorded the widest scope consistentwith the principles and features disclosed herein. The followingdetailed description is to be read with reference to the figures, inwhich like elements in different figures have like reference numerals.The figures, which are not necessarily to scale, depict selectedembodiments and are not intended to limit the scope of the presentteachings. Skilled artisans will recognize the examples provided hereinhave many useful alternatives and fall within the scope of the presentteachings.

The inventors of embodiments of the present invention have developed analternative interconnection design and method of connection replacing oraugmenting the use of rigid connectors, wires, cables or flex circuits.This connection design works effectively with a variety of printedcircuit boards, shapes and sizes. This method of interconnect uses thinsubstrate printed circuit boards specially designed for surface mountand manual soldering to join circuit boards. The connection design isdesirable for several reasons, not only for ease of assembly, but alsofor the structure, appearance and reliability of the connection.Further, because the circuits can be fabricated using conventionalprinted circuit board methods, the interconnect geometry can be easilyadapted for any angle, split, and for a wide variety of pad sizes andspacing. This connection design can be implemented to span shortdistances between boards or to join boards placed end to end. Thisconnector is highly effective in joining printed circuit strips intolarger strips, arrays and matrices, as might be used for SSL lightingapplications.

Embodiments of the present invention described below describeinterconnections providing advantages over both traditional and morerecent methods of interconnect such as the newly introduced FlexRAD™system of continuous connection. Embodiments of the present inventioninclude aspects addressing the strength, reliability and usability ofinterconnects between the semi-flexible substrates in order to producelong strips or continuous reels for ease in fixture assembly.

Embodiments of the present invention provide for a thin board substrate,which makes the connector flexible enough to conform to normalvariations of board thickness, solder height and mechanical mountingheight differences. The thin board substrate allows heat and solder toeasily flow through the connector from top to bottom. An electricalinsulating layer within the thin board is both thin enough to enable ahigh degree of thermal conductivity and is able to maintain high levelsof breakdown isolation. The material chosen for the electricallyinsulating layer enhances thermal conductivity.

The thin board substrate adds flexibility to the connection, reducingstress at the solder joint associated with the use of rigid pins andother types of connectors. This assists in preventing tearing of theprinted circuit board pads on the board when bending stresses areintroduced. The thin board substrate materials and thicknesses assist inhandling solder melt temperatures without delamination or damage. Copperpads on the bottom side of the connector are designed to match the padsof the boards to be connected; in spacing, area and thermalcharacteristics.

Copper pads on a top side can receive heat (e.g., from a soldering iron)and provide a path for conduction through the electrically insulatingsubstrate and/or a plated through hole to the pads on the bottom. Thecopper conductors are used to connect the pads to be mated to theprinted circuit boards. The copper conductors can be thick toaccommodate high currents. Copper conductors can be run on top or underthe connector insulating substrate, depending on requirements forisolation, current carrying capacity and protection.

Embodiments of the present invention provide for copper foils designedto maintain gap distances between connections for electrical isolation.Connections and conductors are protected from damage or shorting bybeing covered by the connector body. Connections and conductors can befurther protected from moisture by the simple addition of an under filllayer of potting material, an encapsulent or an overcoat of pottingmaterial or encapsulant.

Plated holes located at the pad positions, through the connector boardallow solder and heat to flow down into the connection both tofacilitate solder connections and to enable rapid connection. The platedholes located at the pad positions take up excess solder when solderpaste is used to make connections or when solder is applied manually.The plated holes located at the pad positions can be used to storesolder paste for later reflow.

Embodiments of the present invention provide for sealing of solder pastein the holes at the pad positions so the paste remains fresh for lateruse. The sealing may include a thin solder layer, a thin flux layer or athin plastic or metallic peel-off material.

Angled or other geometric patterns in the pad and copper conductorssupport connections for offset or angled printed circuit boards.Multiple pad sets and associated conductor connections allow splittingof conduction paths.

A masking coating over the top and the bottom of the connector board(open at the pads), reduces the opportunity for solder shorts andimprove the appearance of the connector. The masking material can bechosen to match the color and characteristics of the boards beingjointed to minimize the visibility of the connector.

The connectors can be easily formed for vertical step offsets.Connectors onto which other circuits can be used, including pads andgeometries for wire or other conventional types of connectors, as wellas terminations and active circuitry. The connectors can be stackable.Connectors with substrate can extend well beyond pad areas providingmechanical support. Connectors with additional pads can provideadditional strain relief.

The pad geometries may match existing pinned connectors to allow anoption to alternate use of pinned connectors. The thin board can bedesigned to be cut with scissors or a simple shear. Printed lines at thetop of the strip or matrix can show expected cut lines; providingguidance. Copper pads, holes and conductors can be a sufficient spacefrom the cutting location to assure only electrically insulatingsubstrate will be cut.

Embodiments of the present invention provide for intimate contactbetween metal pads with minimal fill layer of solder to increase jointstrength. Larger pads can be used to increase the strength, both becauseof the larger solder contact area, but also because of the larger areasof contact and adhesion between pad and insulating substrate. Largerareas of conductor surrounding exposed, soldered pad apertures increasethe strength both by offering more area for adhesion between conductorsand the insulating substrate, but also because they add to the conductorstructure. The spacing of the pads for maximum array width and heightincreases the joint strength against shear and rotational forces andtorques. A space between pad and edges of the board can be maintained toincrease strength by decreasing leverage and converting stresses intosurface pressures away from the joint.

Embodiments of the present invention disclose increasing the number ofholes leading from the top surface to the pad, which increases thestrength by adding more areas of solder fill. The increased number ofholes also increases the probability of having a better percentage ofsolder fill. The choice of solder type and composition can have animpact on joint strength. Lead baring solders have lower tensilestrength then their lead free counterparts. Higher tensile strengthincreases the fracture strength of the connection.

Embodiments of the present invention provide for the application ofthermal tape or adhesive across the bottom side of the joint to increasejoint strength. The application of potting material or other adhesivesor coatings of the structure adds additional strength to the joint. Inthe areas of board overlap, excluding the conductive pad locations,adhesive can be added to increase joint strength.

Embodiments of the present invention enable connection of two or morecircuit boards to construct various forms, including linear strips andtwo and three dimensional arrays and matrix forms. Embodiments of thepresent invention include construction of flat grids of circuit boards,as well as grids able to be formed around curved surfaces or sharpcorners. In alternate embodiments three dimensional shapes may beformed.

With reference to FIG. 1, a top and cut away view exposing layers of acircuit board with connection pads in an embodiment of the presentinvention is shown. The circuit board 9 can have two electricallyconductive layers 30, 32 with a thin electrical isolating material 31sandwiched in between. The inventors chose the electrically conductivelayers to be 2 oz. copper. The inventors also chose the inner insulatinglayer to be 0.012 inch thick fiberglass composite material. Circuitpaths of various designs can be etched into the top and bottomconductive layers 30, 32 to produce the circuit conductive paths. Platedthrough holes 2 can be added at metal pads 3 and plated through withconductive metal to form a connection between top and bottom. Additionalthin layers of non-conductive solder repelling material 5 (solder masks)can be added to the top and bottom of the board 9 to restrict themovement of solder and protect the circuit paths from the pads 3. Thesolder mask 5 is interrupted to expose conductive pads 4 for mountingelectronic components 13, as well as pads 3 used for board interconnect.On top of the solder mask 10, visible markings may be printed consistingof text and other circuit markings, and special alignment marks 11, 17(FIG. 2A), 28 (FIG. 8C) and 29 (FIG. 8A) or cut marks 33, 34 (FIG. 13C).

In one embodiment the circuit boards 1 (FIG. 2A) and 9 consisted of athin, low thermal mass substrate base material comprised of twoelectrically conductive layers with a thin, electrically isolatingmaterial sandwiched in between. Electrically conductive layers used forproof of concept testing consisted of 2 oz. copper. The thin,semi-flexible circuit boards can be designed with regions of conductorsand pads allowing them to function as connectors, enabling the mating ofone board to another. The circuit board consists of a thin, low thermalmass substrate base material comprised of two electrically conductivelayers with a thin, electrically isolating material sandwiched inbetween. Electrically conductive layers used were of 2 oz. copper. Theinner insulating layer was chosen to be 0.012 inch thick fiberglasscomposite material. Both of these are common to circuit boardfabrication, however generally used for inner layers of a multilayercircuit board, not for circuit board in completion. Circuit patterns 60(FIG. 4B) of various designs were etched into the top and bottomconductive layers to produce the circuit conductive paths. Holes 2 areadded at the pad locations 3 and plated through with conductive metal toform a connection between top and bottom. Additional thin layers ofnon-conductive, solder repelling material 5 (solder masks) were added tothe top and bottom of the board to restrict the movement of solder andprotect the circuit paths away from the pads.

Circuit materials and thicknesses are of a design which allows circuitboards 1, 9 to be cut with a conventional shear or scissors 37 at any ofseveral locations enabling later trimming to length or separation. It isfully contemplated circuit boards could be laser cut as well to obtainindividual circuit strips or arrays. Electrical components, includingLED emitters can be assembled onto circuit boards by conventionalmethods of electronic solder assembly.

Copper conductors can be used for connecting pads 4, 3 to be mated withother electronic components 13. These are etched or formed from theconductive layers 30, 32 described above. These circuit paths can beprinted in almost any pattern commonly used in circuit boards and can bepatterned to receive electronic components 13 such as LEDs 14 orintegrated circuits. The copper conductors can be very thick and wide toaccommodate high currents. In an embodiment 2 oz. copper was used with aconductor width of 0.040 inch to enable a low voltage drop across theconnector when carrying up to 5 amps of current.

It is recognized there may be one or more conductive layers in thecircuit board structure.

Copper foils are designed to maintain gap distances between connectionsfor electrical isolation. In an embodiment, voltage isolations of up to500 V were maintained by maintaining a distance of 0.025 inches betweencopper foils. By increasing the spacing, substantially higher isolationscan be achieved. Copper conductors can be run on top of or under theconnector insulating substrate, depending on requirements for isolation,current carrying capacity and protection.

Circuit boards 1, 9 can incorporate a variety of circuits, includingpads and geometries for wire or other conventional types of connectors,as well as being able to incorporate terminations and active circuitry.The thin circuit board described above is particularly well suitedbecause of its high thermally conductive structure for power and heatcreating circuits. In one implementation, the circuitry for high currentdriver 13 (e.g., one semiconductor #NUD4001 operating at 24 VDC) alongwith a LED string 14 was added to the top side of the board. Both thetop side FIG. 2A and bottom side FIG. 2B of the board were designed withlarge metal (e.g., copper) foils and pads which could translate heatthrough the thin insulating material 31 by effectively creating a largearea for heat transfer from the top copper layer 30 through the lessthermally conductive insulating layer 31 and to the bottom copper layer32.

Connections and conductors can be further protected from moisture by thesimple addition of an under fill layer of potting material or anencapsulent or an overcoat of potting material or encapsulant 24.Potting compounds or conformal coatings are commonly used in theindustry to provide this type of protection. This type of connector isparticularly suitable for these coatings because it is essentially flatwith no recesses or areas which must be protected from contact with thecoatings.

The material chosen for the electrical insulating layer 31 enhancesthermal conductivity. In one embodiment the electrically insulatinglayer 31 was chosen as a high temperature variant of FR4 fiberglass witha glass transition temperature of 170° C., although other materials canbe used. A higher than normal temperature rating of the material isintentionally used to gain more thermal margin allowing for the veryrapid heating (and probable overheating during manual assembly) of thethin boards due to their low thermal mass. Even higher temperaturematerials would be helpful in the case higher melting temperaturesolders are to be used. It is helpful to use an insulating layer 31 bothdurable at high temperatures and as highly thermally conductive aspossible for this construction. Thermal conductivity is helpful for thecases of solder iron or point heat source assembly because it aides inrapid transfer of heat from the top side of the pads 3 to pads 7 below.

With reference to FIG. 2A, a top view of circuit board 1 showselectrically conductive connection pads 3 and plated through holes 2.Conductive pads 4 are designed to accept electronic components 13 andprinted alignment mark 17 as shown. FIG. 2B, show the bottom side of thesame circuit board 1 with additional connection pads 7 and platedthrough holes 2. In this embodiment a large conductive area 6 wasexposed to enable good thermal transfer and heat spreading from top sidecomponents and circuit paths to the bottom side. Optionally, the samearea could be used for additional conductive paths and mounting ofelectronic components.

With reference to FIG. 3, the top side of a second circuit board 9 isshown. Electrically conductive connection pads 8 are designed to matchthe geometry and locations of the bottom side connection pads 7 ofcircuit board 1. Electrical components may be optionally mounted atexposed conductive pads 4 on this circuit board. In this embodiment analignment mark 11 is printed on top of the solder mask 5.

With reference to FIG. 4A, a fully assembled circuit board 12 is shownwith electronic components 13 including LED's 14 mounted onto the board.

With reference to FIG. 4B, two fully assembled circuit boards 12, 16 arejoined together. The lower circuit board 12 alignment mark 11 is used toalign the edge 15 of the upper circuit board 16 so that the connectionpads 8, 7 are in alignment. The upper circuit alignment mark 17 is usedto align the edge of the lower circuit board. It is recognized one orboth of these alignment marks may be of different shapes or forms oromitted in the joining process. It is also recognized mechanicalalignment devices may be used including tooling holes, slots andsighting holes. However, in this embodiment, the inventors chose linearmarks for simplicity and for visual verification of alignment accuracy.

The circuit boards can be overlapped for interconnection (see FIG. 4B,FIG. 5). This is very useful if the connector board contains activecircuitry and it is beneficial to connect multiple boards, such as inthe fabrication of arrays of boards (see FIG. 10). The overlappingconnections are highly advantageous to the assembly of strips consistingof multiple circuit boards (see FIG. 13C). In a practical application,they are used to make long circuit board strips or arrays of solid-statelighting circuits (e.g., high power LED emitters used as the individuallight sources).

Thin board substrate materials and thicknesses are chosen to handlesolder melt temperatures without delamination or damage. Alternatechoices for board insulating material are possible such as Thermagon™ incases where higher temperature resilience and higher thermalconductivity are needed. An embodiment was developed for use with lowertemperature solders (e.g., leaded). Copper pads 7 on the bottom side ofthe upper board 1 are designed to match the pads of the bottom receivingboard 8 in spacing, in area and in thermal characteristics.

With reference to FIG. 5, a side profile view of an overlapping jointbetween boards in an embodiment of the present invention is shown. Inthis embodiment a connection 19 is made by either welding or solderingthe conductive pads 7 from the top board 16 to the bottom boardconductive pads 8 on the bottom board 12. The size of pads 7, 8 factorsinto both the quality of the connection and the mechanical stress theconnection can sustain. Also, by embedding or closely connecting throughholes 2 to pads 7, 8 the mechanical performance is improved. The metalplating and optional solder fill through holes 2 links the top side pads3 to bottom side 8 making the bottom side very difficult to pull off(delaminate) from the insulating layer 31. In the embodiment, holes of0.036 inch diameter are used to promote heat transfer, conduct solderand add enough structure to strengthen the joint. Lapped joints addstrength by adding additional contact area, by reducing leverage, and bychanging certain forces from shearing and tensile to compressive.

The interconnect aspect of FIG. 5 allows for the coupling of circuitboards without a connector or any other device between them.

Plated through holes 2 located at pad positions 3, 7 through circuitboard 16 allow solder and heat to flow down into the connection both tofacilitate solder connection and to enable rapid connection. The rate ofheat transfer being increased by this structure has the additionalbenefit of speeding up solder melting and cooling both during manualsoldering and reflow processing. This saves time and results in better,more repeatable and stronger joints. It is known in the industry fastercooling times result in stronger, more uniform solder joints.

Thin circuit boards can be easily mechanically formed for vertical stepoffsets 21. In experiments run on these boards, bends up to a rightangle could be performed with the conductors (or any foils crossing thebend) on the inside radius of the bend.

The application of tape or adhesive 23, across the bottom side of joint20, further increases joint strength for handling. Viscous tapes act asa spring and dampener to certain stresses, moving forces away from thejoint. The application of potting material 24 or other adhesives orcoatings of structure adds additional strength to joint 20 as well asprotection from mechanical damage and/or moisture (see FIG. 6).

The application of tape or adhesive 23 on the bottom side of the boardassembly 22, allows the assembled strip or array to be directly fastenedto a chassis, enclosure, or heat sink 18 without the use of mechanicalfasteners. In applications for high power LEDs it is particularly usefulto have the tape or adhesive be highly thermally conductive so heat caneasily flow from the circuit boards to the heat sink 18. In oneembodiment, a thermally conductive adhesive tape (e.g., 3M™ product#8810) was applied to the back side. The board assembly 22 can then beadhered to a heat sink 18. The resulting structure maintained excellentheat transfer to the heat sink, which is particularly helpful in highbrightness LED applications.

Intimate contact between metal pads with minimal fill layer of solderincreases strength for joint 19. A thick layer of solder decreasesstrength but adds some flexibility to the joint. Solder has generally amuch lower tensile and shear strength than the conductors it joins.Further, solder tends to have a course crystalline structure and issusceptible to fracturing. A thin layer of solder between copper pads(used the pad material) is much less susceptible to fracturing bothbecause of smaller (or incomplete) crystal formation, and becausestresses are transferred locally to the stronger copper, instead of intothe solder itself.

A number of experiments were conducted to determine solder wetting andflow paths for various pad geometries using the thin connectors insurface mount applications. After it is melted, solder tends to wet tothe metal pads 3 and exposed conductors of printed circuit boards 1 and9. It moves by capillary action to actively fill small gaps and spacesbetween pads 7 and 8, particularly pads in flat surface-to-surfacecontact. If solder was applied in exactly the correct amount, the solderwould simply fill the joints. But even in small excess, the solder wouldpress outside of the pad areas promoting shorts and lower electricalisolation. Holes, recesses or pockets between the pads were tried anddid take up the excess solder. However, the approach was to design inplated holes 2 within the area of the pads 3 and 7 taking up the solderthrough capillary action, effectively pulling excesses into rather thanout of the joint. In the embodiment, the holes were approximately 50% ofthe diameter of the pad, giving ample room for significant variances insolder application.

As a further improvement, plated holes 2 can be used as receptacles forsolder paste so boards 12, 16 could be ready for joining by heat alone.Flux and activating resins, which are commonly incorporated into solderpaste, are needed for high quality solder joints. In one embodiment, thesame plated holes 2 absorb excess solder used to store solder prior tothermal joining. Further, it is recognized the holes can be filled witheither solder paste or separated layers of hard solder and flux resin.In one experiment, solder wire with a core of flux resin was press fitin holes 2 and sheared to match the bottom surface plane of the circuitboard 1. This was another effective way of putting solder and flux intoplated holes 2. Sealing of solder paste in holes 2 at pad positions 3and 7 is helpful so paste remains fresh for later use. Sealing mayinclude a thin solder layer, a thin flux layer or a thin plastic ormetallic peel-off material.

The thin circuit board as described is flexible enough to conform tonormal variations of board thickness, solder height, and mechanicalmounting height differences. Goals for high reliability connectionsinclude robustness, both in mechanical strength and in integrity of theelectrical connection. Several designs and methods were explored andfound to improve both mechanical strength, and in many cases to improvethe electrical connection integrity. By increasing the number of pads 3,7 and 8 used in the connector, mechanical strength was benefited. Simplemultiplication of the number of contacts added to the strength byspreading stress across the added contacts. Redundant parallel contactsreduce electrical resistance and add to the general integrity ofelectrical connection.

Increasing the size of the pads 7 and 8 increases the strength bothbecause of the larger solder contact area, but also because of thelarger areas of contact and adhesion between pad and insulatingsubstrate. In multiple trials, larger pads consistently increased thestrength as measured in pull tests and in bending tests. Larger areas ofconductor surrounding exposed soldered pad apertures increase thestrength both by offering more area for adhesion between the conductorand the insulating substrate, but also because they add to the conductorstructure.

Increasing the distance across a set of pads or span increases the jointstrength against shear and rotational forces and torques. Shear androtational forces (torques) are common during handling of the joinedboards. Of particular use, the assembly of multiple boards into longstrips presents the opportunity to put very high torques on the jointconnection because of the length and lever arm advantage. Preventingdamage due to rotational forces is helpful to having reliable jointswhen the strips are packaged and used in their multiple forms includingstrips and continuous reeled lengths.

By increasing the distance of the pads from the overlapping edges of theboard, the inventors have found a decreased leverage on the individualconnections by converting stresses into surface pressures away from thejoint. By increasing the number of holes 2 leading from top surface tothe pads below, an increase in the strength is discovered by adding morecopper cylindrical connections and rivet like columns of solder filllinking top to bottom. Increased number of holes also increases theprobability of having a better percentage of solder fill between theboards. The choice of solder type and composition can have a directimpact on joint strength. Lead baring solders have lower tensilestrength then their lead free counterparts. Higher tensile strengthincreases the fracture strength of the connection.

Angled or other geometric patterns in the connection pad and copperconductors support connections for offset or angled printed circuitboards. Multiple pad sets and associated conductor connections allowsplitting of conduction paths.

As part of the printed circuit board fabrication process, mask coatingscan be placed over top of circuit boards and the bottom of circuitboards (open at the pads), reducing the opportunity for solder shortsand improving the appearance of the connector or overlapping joint. Inthe embodiments, the mask coating 5 was chosen to match the color andcharacteristics of the boards being jointed so to minimize thevisibility of connection 20.

In the areas of board overlap, excluding the conductive pad locations,adhesive applied between top and bottom board can be added to increasejoint strength. The board connections with overlapping joints can beused to construct elongated strips or arrays of multiple circuit boards(see FIG. 10 and FIG. 13C). Mass parallel construction of long circuitboard strips carrying high intensity LEDs for SSL applications has beenachieved using these connection types.

With reference to FIG. 7, a side profile view of a board to boardconnector joint is shown in an embodiment of the present invention. Thincircuit boards 12 and 16 make connection 20 with an overlapping joint.The circuit boards and connection are flexible enough to conform tonormal variations of board thickness, solder height and mechanicalmounting height differences in many applications. In this embodiment,board to board connection is shown to bend with a radius 25 of less than1 inch. The circuit boards are adhered to a heat sink 18 by double sidedthermal adhesive tape 23, affecting a permanent and highly thermallyconductive bond. The inventors have conceived of several other methodsof attachment, including liquid adhesives, solder or welded bonds,mechanical fasteners, and spring tensioning. In high power LEDapplications, it is particularly helpful to have a good thermalconnection to the heat sink because lower LED device temperaturesimprove brightness, efficiency and increase the expected life.

With reference to FIG. 8A, an alternate embodiment is depicted placingthe location of connection away from the end of the board. The layeredconstruction of the circuit board has been described (see FIG. 1).Conductive pads 3 are shown with plated through holes 2 which passthrough to pads 7 on the underside of the board 26. Printed alignmentmarks 29 provide guidance for connecting overlapping boards. The circuitboard may be pre-assembled with electronic components, such as LEDs 14and associated drive components. FIG. 8B shows the underside of thecircuit board 26. The plated through holes 2 provide electricallyconductive paths from the pads 3 at the top of the board to pads 7 atthe bottom. Thermally conductive pads 6 may be etched or formed into thelower conductive layer enabling heat to better transfer and spread fromthe conductors, pads and components at the top of the circuit board. Thebottom side pads 7 may be electrically isolated from the thermallyconductive pads 6.

FIG. 8C shows the top side of another circuit board 27 in thisembodiment connecting to the circuit board 26. Electrically conductivepads 8 are designed to receive connection from the previously describedboard. Additional alignment marks 28 are used to guide in the assemblyof the two boards.

With reference to FIG. 9, two circuit boards 26 and 27 are joined at aright angle. Alignment marks from the lower circuit board 28 are used tolocate the second circuit board squarely providing vertical guidance.Alignment marks 29 from the upper circuit board 26 align to the edges ofthe lower circuit board 27, providing horizontal guidance. As describedearlier, solder or welding may be used to join the two boards forming areliable joint 100, forming electrical connections between circuitry ofthe two boards.

The inventors conceive circuit boards may be joined at any angle and atany location within the circuit boards in accordance with thisinvention. Further, there are no limits to the number of locations andthe number of circuit boards joined.

With reference to FIG. 10, additional connections are made allowing theconstruction of a two board by two board array 101. The connection joint100 is repeated four times in this embodiment. Additional connectionpads 8 and 3 are indicated at the ends of the boards that can be usedfor connection to other boards or arrays.

The construction of circuit board arrays in accordance with thisinvention are particularly useful in SSL lighting applications becausethey reduce or eliminate wire and mechanical connector attachments andallow LEDs to be placed in specific geometric patterns without requiringas much printed circuit board material be used.

With reference to FIG. 11, construction of larger arrays and grids usingbuilding block arrays and circuit boards is conceived. In thisembodiment, multiple two by two circuit board arrays 101 are connectedto form a larger area array.

With reference to FIG. 12, an alternate embodiment of an array iswrapped around a cylindrical drum 43. In this embodiment, elongatedcircuit boards 41 are joined to additional circuit boards 44 wrappingaround the cylinder 43. The individual boards are joined at connectionjoints 42 similar to those already described.

Circuit boards of various shapes and sizes may be joined to create awide variety of two and three dimensional arrays. The connection designsand methods conceived in the present invention makes it possible toassemble geometries and shapes of circuit board arrays distributingelectronic devices and circuits spatially and enable them to bepositioned and aimed for optimal effectiveness.

An aspect of the utility of constructing strips and arrays of circuitboards is the ability to shape them to size immediately prior toinstallation in a chassis or housing. Long strips and large arrays arepreferable for shipment and stocking purposes, but it is highlydesirable to be able to cut these into smaller strips and arrays fittingthe fixtures and devices they are used in. The inventors have conceiveda system of marking boards, strips and arrays to indicate safe locationsfor cutting. Further, the thin circuit board embodiments described abovecan be easily cut with simple shears or scissors 37 (or any of a varietyof tools or cutting processes).

With reference to FIG. 13A, a printed line is used to mark a safelocation for circuit separation. Conductor patterns 35 etched into theconductive layers of the circuit boards are used to provide power andinterconnect electronic components 13 such as LEDs 14. At locationsdesigned in the circuit cut marks 33, 34 indicate the safe locations forseparating interconnected circuits. In one embodiment, the circuit iscontinuous through the intended cut location. Signal conductors ortraces passing power and optionally control signals will be cut at thesame time as the boards or arrays are separated.

In order to minimize conductor damage and to minimize the opportunityfor short circuits, circuit traces are narrowed at in the immediate area36 of the cut marks 33. Further, the narrower traces are easier to cutbecause they offer less mechanical resistance. In one implementation, 2oz. copper conductors were used with a width of 0.030 inches in the areaof cut. Outside of this area conductors are expanded to improve theircurrent carrying and thermal conduction capability. Outside of this areaare additional components and conductors which could be damaged and arenot intended to be cut or stressed in the cutting process.

It is recognized by the inventors there may not be conductors spanningthe cut marks. There may be one or more power conductors, and one ormore control signals spanning the locations for cut.

With reference to FIG. 13B, a double line cut mark 34 is shown. Thedouble line cut mark 34 has the advantage of showing the boundaries ofthe safe location for cutting the board or array. The inventorsrecognize other ways for indicating safe cutting area including dottedlines, areas of grey or colored printing, tick marks and hatch markscould be used.

With reference to FIG. 13C, circuit separation utilizing the cut marksis achieved with a simple scissors or shear 37. A long strip 40 or arrayis separated into two parts with one part 39 being of desired length,size, and shape for final installation, and the second part 38 eitherbeing the residual or another part ready for final installation.

The inventors conceive the cutting of strips or arrays assembled frommultiple circuit boards may be conducted before or after the addition ofelectronic components onto these boards. Further, additional connectionsand wiring may be needed to complete the assembly. Also, after cutting,the resulting boards, strips, or arrays may again be assembled intoother shapes and combinations using the connection designs describedabove.

While the present invention is directed towards flexible lightingcircuit boards and more directly towards flexible LED circuit boards, itis fully contemplated the present invention could extend to most anytype of circuit board system.

Thus, embodiments of the PRINTED CIRCUIT BOARD INTERCONNECT CONSTRUCTIONare disclosed. One skilled in the art will appreciate the presentteachings can be practiced with embodiments other than those disclosed.The disclosed embodiments are presented for purposes of illustration andnot limitation, and the present teachings are limited only by the claimsthat follow.

The invention claimed is:
 1. An interconnectable circuit board,comprising: a distal end having a first electrically conductive padlocated on a top of the circuit board; a plated through hole on theconductive pad which passes through a conductive layer of the circuitboard and an insulative layer of the circuit board; a secondelectrically conductive pad coupled to the plated through hole; and aproximal end having a third electrically conductive pad attached to thetop of the circuit board.
 2. The circuit board of claim 1, furthercomprising alignment marks for use in aligning circuit boards wheninterconnecting them.
 3. The circuit board of claim 1, furthercomprising cut marks showing locations where the circuit board can becut.
 4. The circuit board of claim 1, further comprising circuit pathselectrically coupled to the electrically conductive pads to provideelectrical interconnectivity between the circuit board and a secondcircuit board.
 5. The circuit board of claim 1, further comprising anon-conductive solder repelling material on a surface of the circuitboard.
 6. The circuit board of claim 1, further comprising a fourthconductive pad for electrically receiving electronic components.
 7. Thecircuit board of claim 1, wherein the insulative layer is sandwichedbetween the conductive layer and a second conductive layer.
 8. Anapparatus for connecting circuit boards, comprising: a first circuitboard having a first electrically conductive pad located on a top of thefirst circuit board; a plated through hole on the first conductive padwhich passes through the circuit board; a second electrically conductivepad coupled to the plated through hole located on a bottom of the firstcircuit board; and a second circuit board having a third electricallyconductive pad on a top of the second circuit board, wherein the firstcircuit board can be placed upon the second circuit board with thesecond conductive pad and third conductive pad aligned to create anelectrical connection between the first circuit board and the secondcircuit board.
 9. The circuit board of claim 8, further comprisingalignment marks located on the top of the first and second circuitboards for use in aligning the first and second circuit boards wheninterconnecting them.
 10. The circuit board of claim 8, furthercomprising cut marks showing locations where the circuit board can becut.
 11. The circuit board of claim 8, wherein the first and secondcircuit boards can be coupled at any angle.
 12. The circuit board ofclaim 8, wherein additional circuit boards could be coupled to the firstand or second circuit board.
 13. The circuit board of claim 8, furthercomprising a fourth conductive pad for electrically receiving electroniccomponents.
 14. The circuit board of claim 8, wherein the first andsecond circuit boards can be coupled together with solder.
 15. Aninterconnectable circuit board, comprising: a first electricallyconductive pad located on a top of the circuit board; a plated throughhole on the conductive pad which passes through the circuit board; and asecond electrically conductive pad coupled to the plated through hole;the second conductive pad capable of being electrically connected to athird electrically conductive pad attached to a top of a secondinterconnectable circuit board.
 16. The circuit board of claim 15,further comprising cut marks indicating safe locations for separatingthe circuit board.
 17. The circuit board of claim 16, wherein circuittraces are reduced in an immediate area of the cut mark.
 18. The circuitboard of claim 17, further comprising a second cut mark adjacent to thefirst cut mark where the area between the first and second cut marks canbe utilized to make a safe cut through the circuit board.
 19. Thecircuit board of claim 18, wherein the circuit board can be cut withscissors.
 20. The circuit board of claim 19, wherein the circuit boardcan be cut to any desired length, size and shape.